Confused

Yusra

I normally gets my periods on the 13th of every month bt on the <a href="https://play.google.com/store/apps/details?id=com.glow.android">Glow app</a>, its telling me im gonna get it on the 9th. Just a bit confused abt the accuracy of my AF. Would i get it on the 13th or the 9th (as according to Glow)

TIA